A twisted nematic (TN) mode, a supertwisted birefringence (STN) mode, an active matrix (AM-LCD) mode and the like have been suggested and put to practical use as a display mode for a liquid crystal display element (LCD). Among them, a supertwisted birefringence (STN) mode in which the orientations of liquid crystal molecules on the upper and lower substrates are twisted to 180-270.degree. (T. J. Scheffer et al.: Appl. Phy. Lett., 45(10), 1021(1984)) is adopted as LCD for personal computers, word processors or the like. More improvement in characteristics have been demanded.
A liquid crystal composition for the liquid crystal display element of the STN mode is required to have the following characteristics:
(1) To show a nematic liquid crystal phase in a wide range of temperature including room temperature, and especially having a high nematic-isotropic phase transition temperature (clearing point). PA1 (2) To have a low threshold voltage in order to reduce a consumed power. PA1 (3) To have a low viscosity (.eta.) in order to reduce a response rate (.tau.) as low as possible. PA1 (4) To be able to take a suitable optical anisotropy (.DELTA.n) in compliance with a liquid crystal display element. PA1 (5) To have a steep voltage-transmittance in order to raise the contrast of a liquid crystal display element, that is, to have a low .gamma. (V.sub.10 /V.sub.90).
Recently, there has been a strong demand for the response of LCD to animations. To respond to animations, a response rate at which LCD is driven should be low. As a response rate is proportional to the viscosity of a liquid crystal composition, a liquid crystal composition having a low viscosity has been required to develop.
As a battery-drive of LCD has become possible, LCD for outdoor use has been developed. To ensure the outdoor use, the range of a nematic liquid crystal phase is required to broaden.
Japanese Patent Kokai 60-19756, 59-190958 and 59-139353 disclose liquid crystal compositions comprising propiolonitrile derivatives, but those compositions contain only liquid crystalline compounds having a positive or weak negative anisotropy of the dielectric constants and have a relatively high viscosity, thus having a slow response rate.
Moreover, the above publications do not disclose the constitutional components and its mixed ratio of the liquid crystal compositions which are required for their characteristics.
Accordingly, there is a continuing need for an improved liquid crystal composition.